A nonalternant azulene-embedded carbon nanohoop featuring anti-Kasha emission and tunable properties upon pH stimuli-responsiveness

Abstract

The insertion of a nonalternant π-system into the skeletons of [n]cycloparaphenylenes ([n]CPPs) can significantly alter their optoelectronic properties. We herein present a nonalternant azulene-embedded carbon nanohoop 1,3-Az[9]CPP. The investigations revealed that 1,3-Az[9]CPP demonstrates unique properties including anti-Kasha emission, reversible pH stimuli-responsiveness and tunable band gaps upon protonation and deprotonation. Importantly, 1,3-Az[9]CPP exhibits a highly selective binding for C₆₀ over C₇₀ (K_C60/K_C70 = 30) in acidic media. Moreover, the molecular structures of 1,3-Az[9]CPP and its fullerene complex C₆₀@1,3-Az[9]CPP were confirmed by single-crystal X-ray diffraction. Notably, 1,3-Az[9]CPP is the first pH-responsive nonalternant all-carbon nanohoop, which may hold significant promising applications in tunable optoelectronic materials.